Optical coherence tomography (OCT) can be used for non-invasive, optical probing of various substances, including but not limited to, skins, body tissues, and organs of humans and animals. In particular, OCT can be used for sensitively detecting spatial variations of backscattered light inside an object and obtaining its tomographic image with high spatial resolution. For example, OCT is often used in ophthalmology to image the retinal morphological features, since the retina has a clear layered structure and the attenuation of light is weak in retina to allow large penetration depth without degrading signal qualities.